Natural tooth shows a complex gradient in translucency, beginning from translucent (enamel) to nearly opaque (dentin) from outer to inner area. Translucency is therefore an important property of dental materials where esthetic matters. The current way to achieve best esthetics in high loaded all ceramic restoration is veneering a high strength ceramic (e.g. Zirconia) material with a low strength high translucent and glass-based veneering ceramic. The veneering step has to be done in most cases by a skilled lab technician and is often time consuming and expensive. Due to the weakness of the veneering porcelain chipping is often observed in several clinical cases.
Another way to produce hard dental tissue saving restorations is using monolithic zirconia without veneering in areas where esthetics play only a minor role (e.g. in the posterior regions of the dental situation).
Zirconia restorations are typically produced via a CAD/CAM process:                A restoration is milled out of a pre-sintered zirconia mill blank.        Afterwards the esthetic of the restoration is increased by colouring the material with a coloring liquid.        The restoration is finished by sintering and polishing.        
WO 2004/110959 relates to a colouring solution for ceramic framework. The solution comprises a solvent (e.g. water), a metal salt and polyethylene glycol having a Mn in the range of 1.000 to 200.000.
WO 00/46168 A1 (corresponding to U.S. Pat. No. 6,709,694 B1) refers to colouring ceramics by way of ionic or complex-containing solutions containing defined concentrations of at least one salts or complexes of the rare earth elements or of the elements of the subgroups. The solution might contain additives like stabilizers, complex builders, pigments and beating additives.
WO 2008/098157 relates to a colouring solution for dental ceramic framework comprising a solvent, a colouring agent comprising metal ions, and a complexing agent, wherein the amount of complexing agent is sufficient to dissolve the colouring agent in the solvent.
WO 2009/014903 relates to a colouring solution for dental ceramic articles, the solution comprising a solvent and a colouring agent comprising rare earth element ions being present in the solution in an amount of at least about 0.05 mol/l solvent and transition ions being present in the solution in an amount of about 0.00001 to about 0.05 mol/l solvent.
CN 102344285 (corresponding to WO 2013/003990) relates to method how to change light transmission of dental zirconia materials. A pre-sintered Y-TZP zirconia is dipped in or brushed with an yttrium containing solution for 2 to 15 min. The zirconia is dried and sintered at 1400 to 1600° C. for 2 h.
Colouring liquids for dental ceramics are meanwhile known in the art.
Examples of colouring solutions are described in U.S. Pat. No. 6,709,694, US 2006/0117989, WO 2009/014903, EP application No. 11177189. The content of these references is herewith incorporated by reference. Theses colouring liquids typically comprise water, metal cations selected from rare earth elements, transition metal and mixtures thereof, and sometimes a complexing agent or further additives like (poly)ethylene glycol. The colouring liquids are typically used for homogeneously colouring dental ceramics and in particular porous dental ceramic framework.
The present invention is intended to improve the known colouring solutions and to extend its use.